A fuel cell is a device for directly converting chemical energy of fuel into electric energy by electrochemical reaction of fuel such as hydrogen or methanol with oxygen. Differently from conventional thermal power generation, a fuel cell exhibits higher efficiency of power generation because it does not undergo Carnot cycle, with less amount of exhaust gas comprising pollutant such as NOx and SOx, without noise during operation. Thus, a fuel cell comes into the spotlight as a clear energy source for next generation.
Fuel cells are classified into polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and so on, depending on the electrolytes used. Among them, a polymer electrolyte membrane fuel cell has lower operation temperature than that of other types, and good efficiency of power generation with property of compactness, so that its usages for power source of electric vehicles, small-scale devices for power generation (such as those for home use), movable power source, power source for emergency, power source for military, and the like are potential.
A polymer electrolyte membrane fuel cell commonly consists of seven layers: a separator/a gas diffusion layer/a fuel electrode/a polymer electrolyte membrane/an air electrode/a gas diffusion layer/a separator. The structure consisting of 5-layer excluding the separators at both ends is commonly referred to as a membrane-electrode assembly (MEA).
As to the operative principle of a fuel cell, fuel such as hydrogen or methanol is uniformly supplied to the fuel electrode first, through the flow field of the separator and the gas diffusion electrode, while air or oxygen is uniformly supplied to the air electrode, likewise to the fuel electrode, through the flow field of separator and the gas diffusion electrode.
In the fuel electrode, fuel is oxidized to generate hydrogen ions and electrons. The hydrogen ions pass through the electrolyte membrane to move toward the air electrode, while the electrons passes through the conduit and load (which constitutes an external circuit) to move toward the air electrode. The hydrogen ions and electrons undergo reduction with oxygen in the air electrode to generate water, which is discharged out of the fuel cell.
Various factors influence the performance of a fuel cell. When considering only a membrane-electrode assembly, the pore structure should be properly controlled to have compatibility of gas diffusion property, ion conductivity and moisturizing property in the fuel electrode and the air electrode, and it is very important to enlarge effective reaction area which is commonly referred to as three-phase boundary reaction area.
Processes for preparing a membrane-electrode assembly essentially include a process of direct applying an ink comprised of catalyst, polymer electrolyte and solvent on a polymer electrolyte membrane (J. Power Sources, 86, (2000), 352); and a process of applying said ink on a gas diffusion electrode such as carbon paper or carbon cloth and compressing it on a polymer electrolyte membrane.
According to the latter process, catalyst particles are apt to be depressed in the porous gas diffusion electrode, so that a homogeneous catalyst layer can be hardly obtained. In addition, mechanical damage may occur due to compression, and the pore structure cannot be appropriately controlled in the coated catalyst layer, so that electrochemical performance cannot be substantially developed.
For directly applying said ink on a polymer electrolyte membrane, commonly employed are screen printing, roller printing, spraying, ink-jet printing, and the like. In case of employing screen printing or roller printing, it is difficult to apply a small amount of ink, while the polymer electrolyte membrane is apt to expand with an increased amount of coated ink, on the other hand. In case of ink-jet printing, the disadvantage indicated is that the coating time is too long to reduce the productivity since the amount of ink to be coated is substantially a trace amount.
In the meanwhile, spraying is advantageous in that the amount of coating can be controlled at will, and the steady amount of coating can be obtained even in case of ink with low viscosity. However, problems have been indicated in that, clogging of the spray nozzle may occur when the solvent is evaporated or dispersion stability of catalyst particles is lowered at the time of coating on the electrolyte membrane, thereby causing reduced process yield and difficulties in controlling the amount of coating.
As an attempt to control the pore structure of the catalyst layer by altering the ink composition and to enhance the electrochemical performance thereby, Japanese Patent Laid-Open No. 2000-353528 discloses that a catalyst-polymer complex is synthesized by using a catalyst carrier having a porous structure of 3-dimensional network and one or more polymer(s), to be used in the ink.
As other attempts, Japanese Patent Laid-Open No. 1996-264190 and J. Power Sources 135(2004) 29 discloses that the polymer electrolyte to be incorporated to the ink is dispersed in solvent to form colloid, and the colloid is adsorbed on the catalyst particles, while Japanese Patent Laid-Open No. 2000-188110 and 2005-108827 disclosed that the molecular weight of the polymer electrolyte incorporated in the ink is lowered. However, those techniques were unsatisfactory to fulfill the purposes of compatibility of gas diffusion property, ion conductivity and moisturizing property, and they might cause environmental problems due to use of excessive amount of organic solvent.